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Insulating Hearth Question

07-21-2006, 01:50 PM

Sorry I don't trust myself in pouring the insulating Hearth. One Idea I had was to buy th concrete backer board and have it sit on top of the stand. Then just leave that there (removing the under supporting structure)

Comment

Sorry I don't trust myself in pouring the insulating Hearth. One Idea I had was to buy th concrete backer board and have it sit on top of the stand. Then just leave that there (removing the under supporting structure)

Does anyone see a problem with that?

The assumption I am going to make is that on top of the block hearth stand you will first pour your reinforced concrete layer. On top of that is the insulation layer. If you have doubts about your ability to mix 6 parts vermiculite to 1 part cement (you mix the water and the cement first and get properly mixed then you add the vermiculite) then the best bet is to go with the ISO panel that Christo suggested.

I am confused about using the concrete backer board (sold at Home Deport/Lowes for use as backing for bathroom walls). Backer Board does not have the structural strength to support a brick oven by itself. The assumption would be that you will use this as the very bottom of the hearth and pour your reinforced concrete on top of it. Remember that when you do this you will need extra support underneath the backer board to support the weight of the concrete until the concrete sets, at least a week. Unlike ¾ plywood which can carry a large load, backer board will flex and fail. Make sure that you have a method to get the concrete to bond with the backer board. On top of this you will put the ISO board.

If you want to get totally away from pouring a concrete hearth and you know how to weld you could make a tubular frame and put in a 1/8 thick mild steel tray and then top that with the ISO board. major weight reduction. Of course you will need to do some engineering calcs to determine minimum wall thickness and determine the load bearing capacity.

Comment

You can definitely use concrete board to "form" the bottom of the hearth, and then just leave it in place. Still, you need to support it underneath with some sort of form to keep it from sagging, or even breaking through. If you go that way, you can leave the concrete hearth and concrete board in place permanently, and don't have to worry about dropping the form down and removing it.

On the second point, we highly recommend using an insulating layer under the oven. While it is true you can put an oven directly on a concrete slab, and it will function, for the small amount of energy and investment it takes, your oven will work much better with an under oven insulating layer.

The basic problem is that heat basically pours through the concrete layer and vents out the bottom. I have test it personally, and have talked with different builders who did it wrong, and had to fix it. We are working with a pizzeria right now to stop heat escaping through their hearth floor.

I would never discourage anyone from building a brick oven -- and any brick oven is better than no brick oven. But having said that, for a little more effort, you can make your oven a lot better.
James

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If I pour the insulating hearth but use plywood underneath can I just lay the plywood on the bricks...with proper support underneath so it won't collapse then leave the plywood there? Will the heat be to much and possibly cause it to catch fire? ...your comment mentioning the heat pours through...

If I poir a 6" hearth then on top of that do a mixture of fireclay and cement under the floor would that be ok... My problem is finding the vermiculite..
I have checked home depot and lowes and only found small bags.

Comment

I think this might be one of those "lost in translation" moments. There is an Italian product called Expanded Clay (Argilla Espansa) that shows up in some of the oven installation guides. It's an expanded clay product, where the "expanded" part serves the same purpose as the "popping" that happens to vermiculite. It's the air holes that provide the insulating value.

I have never seen expanded clay here, haven't worked with it, and I don't know how efficient it is. I have to put that on my to-do list. I put my hands in a bag of it when I was meeting the Artigiano builder. He uses it in his preassembled ovens.